Formation of the grid is the first step requiring improvement. For instance, in self-aligned quad patterning (SAQP), recent changes in resist processing have improved robustness and verticality of the spacer mandrels (Figure 2A). Additionally, he explained that etch smoothing processes, stress engineering of the mandrel, and various reshaping of mandrel profiles have been shown to improve LER and CDU. These methods were combined in a co-optimization flow, and he showed data that demonstrated reduced grid variability (Figure 2B).

Another new way of improving self alignment processes is through the use of “multi-color etch selectivity,” in which the etch process is itself sensitive to the appropriate mandrel layer in a spacer/cut process. This is shown schematically in Figure 3A and Figure 3B.

The topic of selective chemistries leads to selective deposition and etch, and hence to a kind of bottom-up lithography. While top-down lithography has been the mode of operation for many years (Figure 4A), Rathsack explained that one example of bottom-up lithography is self-alignment with selective deposition – metal on metal or dielectric on dielectric. The classic example is, of course, directed self-assembly (DSA) for L/S and Hole pitch multiplication (Figure 4B).

An evaluation of selective metal on metal deposition demonstrated successful growth of Ruthenium (Ru) on Tungsten (W) only (Figure 5A). A Quasi-Atomic Layer Etch (ALE) concept was also shown where a cyclic process of fluorocarbon film adsorption and activation was used in precisely controlled and high selectivity etch processing. Rathsack commented that atomic control of film removal is required for improved selectivity. He showed that a Quasi-ALE process enabled a highly selective and complete etch in a self-aligned contact (SAC) structure—with substantial improvements compared to a conventional reactive ion etch (RIE) (Figure 5B).

Figure 5A. Selective deposition has been used for metal on metal applications (left image). Figure 5B. A Quasi-ALE process enables high selectivity and complete etch for SAC structures.

In summary, Rathsack reminded the audience that edge placement error is the central scaling challenge, and that there are a number of innovative ways to address its many contributors. He stressed that a patterning paradigm shift to self-alignment and bottom-up approaches will be essential in continuing to advance patterning technology.